sstables: ka/la: reader make push_ready_fragments() safe to call many times
sstables: ka/la: reader make push_ready_fragments() safe to call many times

Not a bug fix, just makes the implementation more robust against changes.

Before this patch this might have resulted in partition_end being
pushed many times.

avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla